Thermoplastic resin films, e.g., polyolefin films, polyester films, polyamide films, and polyphenylene sulfide films, are widely used as base films for various purposes, e.g., industrial material uses, magnetic material uses, and packaging uses, because of light weights, small thicknesses, and excellent mechanical properties. Furthermore, in accordance with the trend toward lighter weight, smaller thickness, and further miniaturization, demand for the film is expected to grow in various areas.
Most of all, biaxially oriented polyester films have excellent properties with respect to the dimensional stability, mechanical properties, thermal resistance, electrical properties, and the like, and therefore, are widely used as base films for many purposes, for example, magnetic materials, e.g., magnetic tapes typified by VHS tapes, audiotapes, and backup tapes of computer data and cards, such as prepaid cards, IC cards, and optical recording cards; packaging materials; electrically insulating materials; various photographic materials; graphic arts materials; and label materials.
However, since the polyester is an insulating resin, in general, there is a problem in that biaxially oriented polyester films have no antistatic property unless being further treated. As a result, dust tends to settle on the biaxially oriented polyester film if no treatment is performed, and products composed of the biaxially oriented polyester film tend to become into intimate contact with each other due to electrification, so that ease of handling is deteriorated. Furthermore, electrostatic problems occur in that, for example, an electric shock and explosion occur due to electrostatic discharge, and precision electronic circuit material elements and the like are broken due to electrostatic discharge.
Consequently, researches have been previously conducted in order to impart the antistatic property to polyester films by various methods.
Examples of known methods for preventing electrification include incorporation of an antistatic agent into a base film and impartation of an antistatic property to a base film surface. For example, methods in which a polyester resin is mixed with an antistatic agent and is applied, as described in Japanese Unexamined Patent Application Publication No. 60-141525 and the like, and methods in which a styrene sulfonic acid copolymer is applied, as described in Japanese Unexamined Patent Application Publication No. 61-204240 and the like, are methods for preventing electrification through the use of ionic conduction type antistatic agents. In these methods, an electrical conduction mechanism depending on adsorption of water in air due to ions is used and, therefore, these methods have dependence on humidity. In particular, the humidity dependence is significant when a low-molecular weight type antistatic agent is used and, therefore, with respect to the product quality, serious problems occur in that, for example, no antistatic property is attained under an environment in which the humidity is low as in winter and the like.
In methods described in Japanese Unexamined Patent Application Publication No. 7-101016, Japanese Unexamined Patent Application Publication No. 7-330901, and the like, a layer of a polyaniline-based conductive agent is provided on a surface of a polyester film, and the polyaniline-based conductive agent to be used is an electron conduction type antistatic agent. The conduction mechanism thereof is based on conjugated electrons and, therefore, has no dependence on humidity. However, the polyaniline-based conductive agent is green in the condition of being subjected to doping, and there are problems in that, for example, an outward appearance of a product is not preferable, and uses are restricted.
In methods described in Japanese Unexamined Patent Application Publication No. 9-152723, Japanese Unexamined Patent Application Publication No. 11-278582, Japanese Unexamined Patent Application Publication No. 7-329250, and the like, a layer of tin oxide-based conductive agent doped with antimony is provided on a polyester film surface. These are methods for preventing electrification through the use of an electron conduction type antistatic agent. The conduction mechanism thereof is based on conjugated electrons and, therefore, has no dependence on humidity. However, in order to exhibit the conductivity, the tin oxide-based antistatic agent is in need of a doping agent containing a hazardous heavy metal, e.g., antimony. Furthermore, when a granular antistatic agent typified by the tin oxide-based antistatic agent is applied to an in-line coating method in which coating, stretching, and a heat treatment are performed during a film formation step, no follow-up property to stretching is exhibited and, therefore, problems occur in that, for example, cracks occur in the coating film due to stretching, the coating film is whitened or the coating film becomes brittle, and no scratch resistance is exhibited.
In methods for preventing electrification through the use of other electron conduction type antistatic agents described in Japanese Unexamined Patent Application Publication No. 1-313521, Japanese Unexamined Patent Application Publication No. 6-295016, and the like, impartation of the antistatic property by a polythiophene-based conductive agent is proposed. However, an adequate antistatic property is not attained by, for example, a method for preventing electrification in which a coating solution containing the polythiophene-based conductive agent and a latex polymer is applied. In order to exhibit a high level of antistatic property, large amounts of polythiophene-based conductive agent must be added. Consequently, when this method is applied to the in-line coating method, there are problems in that the coating film has significantly poor transparency, and the coating film is whitened, in a manner similar to that in other electron conduction type antistatic agents. Therefore, this method is not worthy of practical use.
Protective films will be described below.
In recent years, the changeover from so-called cathode-ray tube televisions to liquid crystal displays have been actively pursued in accordance with a revolution in displays. In particular, with respect to the liquid crystal display, there are steps of processing and mounting of optical sheets, e.g., polarizers, serving as primary constituents, and a protective film is used in order to protect a surface. An adhesive and the like is applied to or laminated on a transparent film, e.g., a polyethylene film, a polypropylene film, or a polyester film, which is used for protection, in order to bond the film together with an optical sheet, e.g., a polarizer. The resulting film is used as the protective film.
After mounting of a liquid crystal display and the like is completed, the protective film is peeled and removed. When the protective film is peeled, a so-called peeling electrification phenomenon occurs, and there is a problem of adhesion of dirt due to static electricity. If dirt is adhered due to static electricity, for example, it is difficult to discriminate between defects of a liquid crystal component itself and defects due to dirt adhered to the surface with respect to the inspection of products, and the inspection is not conducted smoothly. As a result, significant problems occur in the manufacturing process. In particular, with respect to recent high-precision display and the like, besides the problem of the adhesion of dirt due to static electricity, a problem of breakage of electronic elements of a display occurs due to peeling electrification.
On the other hand, a protective film prepared by incorporating an antistatic agent to a polyethylene film or a polypropylene film has low transparency. Since the transparency of the protective film is poor, there are problems in that, for example, the inspection accuracy is deteriorated in the inspection of defects of products after a liquid crystal display and the like are mounted, and the inspection takes much time.
With respect to even a protective film made of a polyester film having excellent transparency, a non-treated film has no antistatic property and, therefore, many troubles, e.g., adhesion of dirt, occur due to electrification. In order to overcome this, researches have been conducted on a polyester film incorporating an antistatic agent and a polyester film coated with an antistatic agent. However, any satisfactory film has not been achieved.
Furthermore, cover tapes and carrier packages will be described.
In recent years, surface-mounting chip-shaped electronic components, e.g., ICs (integrated circuits), transistors, diodes, and capacitors, are packaged with a carrier package composed of a carrier tape successively provided with pockets or the like adjusted to the shape of the electronic components by embossing in order to store them and a cover tape sealing the carrier tape through heat seal or an adhesive, the carrier package is taken up into the shape of a reel, and is supplied.
When the surface-mounting chip-shaped electronic components are used in practice, they are carried, while being in the state of the reel, to a manufacturing step in which the surface-mounting chip-shaped electronic components are used, and products are manufactured continuously on a production line while the cover tape is peeled off the carrier package, in which the carrier tape is sealed with the cover tape, and the electronic components are taken out.
When the cover tape is peeled off, problems occur in that the electronic components are accidentally released and the electronic components are scattered or that discharge occurs between an electrified tape and an electronic component and the electronic component is electrically broken.
Beside the peeling electrification, in a step of storing an electronic component in a carrier tape, followed by sealing, it is known that a phenomenon of frictional electrification occurs due to friction between the electronic component and a carrier roll, a heating bar used for heat seal, and the like. Since the surface-mounting chip-shaped electronic component is a small electronic component, for example, in the case where the winding is performed into the shape of a reel having a very small width on the order of 5 mm and a length on the order of several thousand meters, if the frictional electrification phenomenon occurs, problems come up, wherein straying occurs and, thereby, the winding into the shape of a reel cannot be smoothly performed, or an electronic component in a stored state is electrically broken, in a manner similar to that in the above-described peeling electrification.
In general, when packaging into the carrier package is completed, inspections of the electronic components are performed from the cover tape side, wherein the inspection items includes mixing of different types, bending of lead terminals, error in packing direction of IC chips, and the like. In order to readily perform these inspections, the cover tape is strongly required to have transparency. For example, a transparent polyester film, a transparent polyolefin film, and the like, each having high transparency, have been used directly.
Under the above-described circumstances, research is conducted on impartation of the antistatic property to the carrier tape side in order to overcome the electrification problem.
In many cases, various resins, e.g., polystyrene, are used as carrier tapes. In order to reduce electrification of the carrier tape itself, for example, carbon or the like serving as an antistatic agent is incorporated, or paper or the like is used as an antistatic agent with respect to inexpensive types. However, any carrier tape having satisfactory performance has not been attained until now.